Web Services
Related Standards
Introduction to Web Services
Ioannis G. Baltopoulos
Department of Computer Science
Imperial College London
CERN School of Computing (iCSC), 2005
Geneva, Switzerland
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
1 Web Services
Fundamental Concepts
Architectures & eScience example
2 Related Standards
XML
SOAP
WSDL
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
Fundamental Concepts
Architectures & eScience example
Distributed Computing Technologies
Historic Review (20 years in 5 minutes!)
CORBA (OMG)
It is standards-based, vendor-neutral, and language-agnostic.
Very powerful but limited however by its complicated way of
utilizing the power and flexibility of the Internet.
DCOM (Microsoft)
Distributed Computing platform closely tied to Microsoft
component efforts such as OLE, COM and ActiveX.
RMI (Sun Microsystems)
Java based effort which doesn’t play well with other
languages. The J2EE platform integrated RMI with IIOP.
Web Services (W3C)
Web services are more of an evolution than a revolution
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
Fundamental Concepts
Architectures & eScience example
What is a Web Service?
Definition
A Web Service is a standards-based, language-agnostic software
entity, that accepts specially formatted requests from other
software entities on remote machines via vendor and transport
neutral communication protocols, producing application specific
responses.
Standards based
Language agnostic
Formatted requests
Remote machines
Vendor neutral
Transport neutral
Application specific
responses
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
Fundamental Concepts
Architectures & eScience example
Benefits of Web Services
Loosely Coupled
Each service exists independently of the other services that
make up the application. Individual pieces of the application
to be modified without impacting unrelated areas.
Ease of Integration
Data is isolated between applications creating ’silos’. Web
Services act as glue between these and enable easier
communications within and across organisations.
Service Reuse
Takes code reuse a step further. A specific function within the
domain is only ever coded once and used over and over again
by consuming applications.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
Fundamental Concepts
Architectures & eScience example
Web Services Architectures
The simplest Web Service System
The simplest Web service system has two participants:
A service producer (provider)
A service consumer (requester).
The provider presents the interface and implementation of the
service, and the requester uses the Web service.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
Fundamental Concepts
Architectures & eScience example
Web Services Architectures
A Service Oriented Architecture (SOA)
A more sophisticated system:
A registry, acts as a
broker for Web services.
A provider, can publish
services to the registry
A consumer, can then
discover services in the
registry
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
Fundamental Concepts
Architectures & eScience example
e-Science example
Web Enabled Telescope Access Requirements
In the context of eScience and observatories, there are several
requirements from a distributed astronomical system.
For example,
different people need access to subsets of the same data,
data needs to be archieved for future use,
same functionality implemented using different technologies,
certain authorities authorize the use of resources,
others are responsible for cataloging available resources.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
Fundamental Concepts
Architectures & eScience example
e-Science example
Web Enabled Telescope Access
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
eXtensible Markup Language (XML)
Definition
The eXtensible Markup Language (XML) is a W3C
recommendation for creating special-purpose markup languages
that enable the structuring, description and interchange of data.
A simplified subset of SGML capable of describing many
different kinds of data for any imaginable application domain.
It facilitates the sharing of structured text and information in
databases and across the Internet.
Languages based on XML are themselves described in a formal
way, allowing programs to modify and validate documents in
these languages without prior knowledge of their form.
Separate syntax from semantics.
Inherently supports internationalization (Unicode) and
platform independence.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Building Blocks
Elements
The pairing of a start tag and an end tag.
Attributes
A name-value pair that is part of a starting tag of an Element.
Processing Instructions
Special directives to the application that will process the XML
document.
Comments
Messages helping a human reader understand the source code.
Character Data
Characters (in a specific encoding)
Entities
Whitespace
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Elements
Formal Definition & Rules
Definition
The term element
is a technical name
for the pairing of a
start tag and an
end tag in an XML
Document.
Production Rule
〈element〉 ::= 〈EmptyElement〉
| 〈STag〉 〈content〉 〈ETag〉
〈STag〉 ::= ‘<’ 〈Name〉 〈Attribute〉? ‘>’
〈ETag〉 ::= ‘’
〈EmptyElement〉 ::= ‘<’ Name 〈Attribute〉? ‘/>’
XML Elements must be strictly nested!
Element names can include letters, the underscore, hyphen
and colon; they must begin with a letter.
Element names are case sensitive!
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Elements
Some right & wrong examples
Example
Welcome!
Wrong Examples
Nesting Problem
<.wrong.element> some text
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Attributes
Formal Definition & Rules
Definition
The term attribute(s) refers to a theoretically arbitrary number of
name-value pairs that can be included in the starting tag of an
XML element.
Production Rule
〈STag〉 ::= ‘<’ 〈TagName〉 〈Attribute〉? ‘>’
〈Attribute〉 ::= AttrName ‘=’ Value
The value part of the attribute
has to be quoted.
Attribute names starting with
xml:are reserved by the XML
specification.
Example
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Processing Instructions
Definition, Rule & Example
Definition
A special directive to the
applications processing the
XML documents.
Production Rule
〈PI 〉 ::= ‘’
Example
The PI Target keyword is meaningful to the processing
application and hence could be different between applications.
Everything between the PI Target and the closing question
mark is considered the contents of the processing instruction.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Comments & Character Data
Definition, Rule & Example
Comment A message that helps the human reader understand the
program and the processing that takes place at a particular point
of the source code.
Production Rule
〈Comment〉 ::= ‘’
Character Data
Encoding: All characters in an XML document must comply
with the document’s encoding; those outside the encoding
must be escaped and are called character references.
Whitespace: Whitespace can be treated as either significant
or insignificant. Most XML applications care little about
whitespace.
Entities: Like character references, they are predefined escape
sequences that map to specific characters.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
An XML Document
Putting it all together!
Hey, I’m at the iCSC!
Photo from Geneva
0100100001010001001010010
An XML Document
consists of:
Optional prolog
A root element
Comments
Processing
Instructions
But...
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Some Problems
And how we solved them!
The problems in the previous example relate with the:
Physical Structure of the document
Well formedness (Parsers)
Logical Structure of the document
Validity (Schemas). Semantics of the elements?
Element Name clashes between Documents
Namespaces
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Namespaces
Motivating the Problem
Solve the problem of recognition and collision of elements in an
XML Document.
Recognition
How does an XML processing application distinguish between
the XML elements that describe the message and the XML
elements that are part of a Purchase Order?
Collision
Does the element description refer to attachment descriptions
in messages or order item descriptions? Does the item
element refer to an item of attachment or an order item?
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Namespaces
Detailing the Solution
XML Namespaces uses Uniform Resource Identifiers for uniquely
qualifying local names. As URIs can be long and typically contain
characters that arent allowed in XML element names, the process
of including namespaces in XML document involved two steps:
A namespace identifier is associated with a prefix, a name
that contains only legal XML element name characters with
the exception of the colon (;)
Qualified names are obtained as a combination of the prefix,
the colon character, and the local element name
Qualified Name = Namespace Identifier + Local
Name
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
A Namespaces XML Document
A Purchase OrderLaptop Computer1300 GBP
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Namespaces
A couple more last things
Default Namespaces
Adding a prefix to every element in the document decreases
readability and increases document size. Therefore, XML
Namespaces allow us to use a default namespace in a
document. Elements belonging to the default namespace
don’t require prefixes.
Namespace prefixed attributes
Attributes can also have namespaces associated with them.
The desire to extend the information provided by an XML
element without having to make changes directly to its
document type.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
XML Schema
An XML Schema enables the following:
Identification of the elements that can be in a document
Identification of the order and relation between elements
Identification of the attributes of every element and whether
they’re optional or required or have some other special
properties
Identification of the datatype of attribute content
Think of it as an elaborate UML Class diagram where classes only
have field and no methods.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Simple Object Access Protocol (SOAP)
What’s the big deal?
Definition
SOAP is an industry accepted W3C specification for a ubiquitous
XML distributed computing infrastructure.
A mechanism for defining the unit of
communication.
A mechanism for error handling.
An extensibility mechanism
Lives above the transport layer of OSI
Simply put its a mechanism that allows
the transmission of XML documents,
regardless of transport layer protocol.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
SOAP Messages
Logical & Physical Structure
The root element of a SOAP message
is the Envelope element.
It contains an optional Header
element and the required Body
Elements called Faults can be used
to describe exceptional situations.
It can contain optional Attachments in
MIME encoding for exchanging binary
data.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
SOAP Example
Structure of a real XML SOAP Message
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
SOAP Message Transmission
Message delivery path using Intermediaries
The SOAP Message Transmission involves three main roles:
The SOAP Sender creates and sends a SOAP Message to an
ultimate SOAP Receiver.
One or more optional SOAP Intermediaries can be
positioned to intercept messages between the the sender and
the receiver. They can perform filtering, logging, catching etc.
The SOAP sender’s intended destination is called the
Ultimate SOAP Receiver.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Web Services Description Language (WSDL)
Web Services Description Language (WSDL) is an XML format for
describing all the information needed to invoke and communicate
with a Web Service. It gives the answers to the questions Who?
What? Where? Why? How? A service description has two major
components:
Functional Description
Defines details of how the Web Service is invoked, where it’s
invoked. Focuses on the details of the syntax of the message
and how to configure the network protocols to deliver the
message.
Nonfunctional Description
Provides other details tha are secondary to the message (such
as security policy) but instruct the requestor’s runtime
environment to include additional SOAP headers.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
WSDL Document Structure
The 6 basic building blocks
A WSDL Document is a set of definitions with a single root
element. Services can be defined using the following XML
elements:
Types, think Data Type
Message, think Methods
PortType, think Interfaces
Binding, think Encoding Scheme
Port, think URL
Service, many URLs
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
PortType Element
Definition and Usage
Definition
The portType element describes the interface to a Web Service
A WSDL Document can contain zero or more portType
A portType element contains a single name attribute.
Naming convention nameOfWebService PortType
A portType contains one or more operation elements, with a
name attribute can contain input, output and fault elements
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
PortType Element
Example
Example
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Message Element
Definition and Usage
Definition
A message is a collection of parts; intuitively a part is a named
argument with its type. A message is a collection of these parts.
A WSDL document can contain zero or more message
elements.
Each message element can be used as an input, output or
fault message within an operation .
The type attribute of part can be any standard data type
from the XSD Schema or a user defined one.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Message Element
Example
Example
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Types Element
Definition and Usage
Definition
Custom user data types defined in an abstract way.
The default type system in WSDL is the XML Schema (XSD)
A WSDL document can have at most one types element.
The types element can contain simpleType or complexType.
At the lowest level elements intuitively named (again!)
element are defined with a name and a type attribute.
NOTE! The diagram bellow is incomplete! This is considered an advanced topic and for more information you
should look at data modelling using the XML Schema.
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Types Element
Example
Example
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Binding Element
Definition and Usage
Definition
The binding element specifies to the service requester how to
format the message in a protocol-specific manner.
Each portType can have one or more binding elements
associated with it.
For a given portType the binding element has to specify an
messaging and transport pair. (SOAP/HTTP, SOAP/SMTP,
etc).
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Binding Element
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Port Element
Definition, Usage & Example
Definition
The port element specifies the
network address of the endpoint
hosting the Web Service.
It associates a single
protocol-specific address to an
individual binding element.
Ports are named and must be
unique within the document.
Example
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Service Element
Definition and Usage
Definition
The service element is a collection of related port elements
identified by a single service name.
A WSDL Document is allowed to contain multiple service
elements, but conventionally contains a single one.
Each service must be uniquely named.
The naming convention is GeneralInfoService
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Service Element
Example
Example
Ioannis G. Baltopoulos Introduction to Web Services
Web Services
Related Standards
XML
SOAP
WSDL
Concluding Remarks
In this first lecture we saw
the position of Web Services within the Distributed
Computing Environment.
the XML primitives and touched upon Namespaces and
Schemas.
how SOAP is used for transferring platform and language
independent messages between software entities on different
hosts.
how to describe Web Services using WSDL.
...now...GO FOR COFFEE!
Ioannis G. Baltopoulos Introduction to Web Services
